Angular - ViewChild(Directive) 在指令中返回未定义

Question

Angular V7.x

I am creating a Directive that adds a dynamic component to the specified container. I am using another Directive to mark as an insertion point but, ViewChild() returns undefined no matter what. Even if it's accessed in ngAfterViewInit . Angular Docs example accesses ViewChild in ngOnInit , though.

alert.directive.ts

import { Directive, ViewChild, TemplateRef, ViewContainerRef, Input } from '@angular/core';
import { AlertHostDirective } from './alert-host.directive';

@Directive({
  selector: '[appAlert]'
})
export class AlertDirective {
  @ViewChild(AlertHostDirective) alertHost;

  constructor(
    private templateRef: TemplateRef<any>,
    private viewContainer: ViewContainerRef
  ) {}

  ngAfterViewInit() {
    console.log(this.alertHost); // undefined
  }

  ngOnInit() {
    console.log(this.alertHost); // undefined
  }

  @Input() set appAlert(name: string) {
    this.viewContainer.createEmbeddedView(this.templateRef);
  }
}

alert-host.directive.ts

import { Directive } from '@angular/core';

@Directive({
  selector: '[appAlertHost]'
})
export class AlertHostDirective {

  constructor() { }

}

form-signup.html

<form *appAlert="'signupForm'" class="form-signup" (submit)="onSubmit($event)" [formGroup]="signupForm">
      <h2 class="mb-4">Sign Up &mdash; it's free</h2>
      <ng-template appAlertHost></ng-template>
      ... (more content)
</form>

StackBlitz Link: VIEW ON STACKBLITZ

Answer 1

由于您的 AlertHostDirective 不是在组件的 HTML 模板中使用，而是在表单的开始和表单的结束标记之间使用，因此您需要使用ContentChild来访问您的指令。